1. Field of the Invention
The present invention relates to ultrasonic beamforming, more specifically the present invention relates to an analog store, digital read (ASDR) ultrasound beamforming system and associated method.
2. Background of the Invention
There are number of areas in the electronics field in which analog memory devices are being used successfully such as digital storage oscilloscopes, and in the physics field X-ray and charged-particle tracking applications. Some early predecessors of this technology can be traced to digital oscilloscopes and waveform capturing devices based on Fast-In-Slow-Out (FISO) principle such as one described in U.S. Pat. No. 4,271,488 entitled “High-Speed Acquisition System Employing An Analog Memory Matrix” or in U.S. Pat. No. 4,833,445 entitled “FISO Sampling System”. These patents are incorporated herein by reference and the latter patent depicts the fast, high resolution FISO system, while the former describes an acquisition system that uses an analog memory matrix built of sample-hold cells arranged in rows and columns to form an M×N matrix that may be implemented on a single integrated-circuit (IC) chip.
The idea of a matrix analog memory device on IC was further developed by Stewart Kleinfielder who produced a range of multichannel transient analog waveform digitizer chips used to capture data from detectors in neutrino physics experiments, as well as by other contributors (for example, see Kleinfelder, S. A., “A 4096 Cell Switched Capacitor Analog Waveform Storage Integrated Circuit”, IEEE Transactions on Nuclear Science, NS-37, No. 1, February 1990.; and Kleinfelder, S. A., “Advanced Transient Waveform Digitizers,” SPIE Particle Astrophysics Instrumentation Proc., v. 4858, pp. 316-326, August 2002.) Additional prior art representing informative background can be found in U.S. Pat. No. 5,722,412 entitled “Hand Held Ultrasonic Diagnostic Instrument”; U.S. Pat. No. 6,126,602 entitled “Phased Array Acoustic Systems with Intra-Group Processors”; U.S. Pat. App. Pub. No. 2008-0262351A1 entitled “Microbeamforming Transducer Architecture”; U.S. Pat. App. Pub. No. 2010-0152587A1 entitled “Systems and Methods for Operating a Two-Dimensional Transducer Array”; and U.S. Pat. APP. Pub. No. 2011-0213251A1 entitled “Configurable Microbeamformer Circuit for an Ultrasonic Diagnostic Imaging System.” See also Haller, G. M.; Wooley, B. A., “A 700-MHz switched-capacitor analog waveform sampling circuit,” IEEE Journal of Solid-State Circuits, v. 29(4), pp. 500-508, April 1994. The above identified patents and published patent applications are incorporated herein by reference.
In medical diagnostic ultrasound, there were a number of attempts to use analog memory for ultrasound signal beamforming, notably U.S. Pat. Nos. 6,500,120 and 6,705,995, which are incorporated herein by reference. The process of ultrasound imaging, such as in medical diagnostic, begins with sending specially constructed ultrasonic signals (pulses, waves or wave packets) into the subject, e.g., tissues in medical diagnostics (or turbine blades for jet engine inspection, etc.) The pressure pulse propagates in depth while attenuating and scattering on the acoustic impedance interfaces (such as a boundary between different tissues) along the way. These scattered echoes are picked up by the receiving ultrasound array and from this data the tissue composition along the pulse propagation path is reconstructed as a single scan line. Then, the next pulse is sent into a different direction and the process of receiving scattered (or attenuated as in transmission tomography) ultrasound signals back to the sensor array, and the interpretation of the results is repeated until a required 2-D slice (B-mode frame) or a 3-D volume is assembled out of separate scan lines.
In order to increase the spatial and contrast (magnitude) resolution of a signal coming from the certain spatial location within the tissue, the ultrasound array needs to be focused on that location. Thus, in the course of pressure pulse propagation in the tissue, the receiving array needs to constantly shift its focus following the pulse current position. Therefore, one of the first steps in processing the raw data is called beamforming in which signals coming to different elements of the array are time-shifted before they will be added to one another. As a rule, the beamforming applies to both, transmit and receive signals.
FIG. 1 illustrates the first method used in forming ultrasound images, also known as analog beamforming Generally, the ultrasound imaging device consists of an ultrasonic array 106 divided to a number of independent elements 107 or channels (typically to 64 or 128 elements in linear or curved 1D array). During the transmit stage of interrogation, the transmit beamformer sends variably delayed electric pulses to the elements of the ultrasound array 106. The relative delays between the signals is constructed in such a way that ultrasonic pulses emitted by elements 107 of the array 106 would arrive to the predetermined spatial point 100 (focal point P) simultaneously, with their phases aligned to achieve a coherent summation of wavelets coming from all elements 107 of the array 106. This wave would scatter at the point 100 and part of this spherical scattered wave would travel back to the elements 107 of the array 106. Each element 107 would convert pressure variations in the incoming wave into the voltage variation output 108. The portion of this scattered wave that reaches a face surface of an array element 107 can be seen as a wavelet 102 that travels along the ray 104 that connects the scattering point 100 and the face of the element 107. Depending on the mutual position of the scattering point 100 and the specific element 107 of the array 106, the path 104 would vary from the shortest one equivalent to radius R0 105 to the longest one. The spatial difference AD; between the shortest path 105 and path from the point 100 to the i-element of the array 106 translates into the time delay Δti between the arrivals of signals 108. The task of the receive beamformer is to modify the time differences between the signals 108 from all elements 107 participating in beamforming and sum them in accordance with the directions of the beamforming algorithm. For example, such a beamforming algorithm may require removing the time delays Δt from all arrived signals and sum such processed signals (delay-sum algorithm), in effect focusing the array to the point P. It can be seen that workings of transmit and receive beamformers are mutually reciprocal, thus, describing the works of the receive beamformer is also a description of the solutions for the transmit beamformer.
The ways received signals are processed define the type of the beamformer. The analog beamformer shown on FIG. 1 was a first type of the beamformer used to process ultrasound signals. In it signals 108 were first amplified by voltage controlled amplifier (VCA) 110 to compensate the signal attenuation, then, a delay circuit 112 was used to time shift the signals to compensate the delays in arrival, then such aligned signals 114 were summed in analog summing circuit 116 and the output signal 118 was digitized by analog-digital converter (ADC) 120 producing output digital signal 122 that was stored in memory and used by the back end processor to reconstruct B-mode or Doppler images. The advantage of such design is the simplicity of the hardware. The disadvantages include poor time discrimination and low refresh rate of the analog design elements 112 (no dynamic beamforming) as well as irreversibility of the beamforming process such that only one beamforming algorithm can be applied to the captured signals.
The second common type of the beamformer used in ultrasound imaging is commonly known as the digital beamformer (see FIG. 2). In the digital beamformer, voltage signals 108 from the elements of the array 106 are amplified by the voltage controlled amplifier (VCA) 110 to compensate the signal attenuation, then, the signal in each channel is digitized at a certain sampling rate by channel ADC 124 that outputs digitized signal to the memory or Firs-In-First-Out (FIFO) registers where signals are shifted in accordance with the beamforming algorithm (for example such that to remove arrival delay Δt), then such processed digital data 128 from each participating channel are summed by digital summator 130 and output data 122 are written to the memory for further processing. The advantages of digital beamformer, such as shown in FIG. 2 are its speed and precision which allows implementation of the dynamic beamforming and the possibility of realization of multiple beamforming strategies on the same data volume. The disadvantage is complexity of the hardware; manifesting in larger hardware size, higher cost, and higher power consumption (heat generation).
For the reasons of clarity, the beamforming schematic for analog and digital beamformers shown on FIGS. 1 and 2 was simplified by removing the multiplexing stage. However in reality, as known to those of ordinary skill in the art, having the number of processing channels be equal to the number of the arrays' elements is a very expensive proposition. Thus, the array can have 64, 128, 256 or greater number of elements but the beamformer would have typically 32 or 64 channels and an analog multiplexing circuitry that would select elements of the array 106 into the current aperture. Also for the same reasons, cable and signal connectors that connect elements of array 106 to the analog front-end electronics are not shown, even though they do affect the cost and signal quality of the system.
From the description of the beamforming process it can be seen that the signal coming from the output of the array element 107 is processed independently from the signals coming from the other elements up to the output of the beamformer where all of the signals are combined. Thus, this text will refer to this signal path from the element 107 to the input of summator 116, 130 (or 136) as a “signal path” or “beamforming channel” or simply as “channel” 109.
As further background the international search report in PCT/IB2014/000281 identified that publications US2012-1433059 and US2010-0331689 and U.S. Pat. Nos. 8,545,406 and 8,317,706 were of general interest to the present invention and these disclosures are incorporated herein by reference.
There remains a need in the art to reduce the size and power requirements of diagnostic ultrasound imaging and to utilize beamforming architecture to accomplish this goal.